In process measurements and automation technology, for measuring physical parameters, such as, for example, the mass flow, the density and/or the viscosity of media flowing in pipelines—for instance, an aqueous liquid, a gas, a liquid-gas mixture, a steam, an oil, a paste, a slurry or another flowabie material—such in-line measuring devices are often used, which, by means of a measuring transducer of the vibration type through which medium flows, and by means of a measuring and operating circuit connected thereto, effect reaction forces in the medium—such as, for example, Coriolis forces corresponding to a mass flow, inertial forces corresponding to a density of the medium and/or frictional forces corresponding to a viscosity of the medium, etc.—and, derived from these, produce a measurement signal representing the particular mass flow, viscosity and/or density of the medium. Such measuring transducers, especially measuring transducers embodied as Coriolis mass flow meters or Coriolis mass flow/densimeters, are described at length and in detail in, for example, EP-A 1 001 254, EP-A 553 939, US-A 2002/0157479, US-A 2006/0150750, US-A 2007/0151368, U.S. Pat. No. 4,793,191, U.S. Pat. No. 5,370,002, U.S. Pat. No. 5,796,011, U.S. Pat. No. 6,308,580, U.S. Pat. No. 6,415,668, U.S. Pat. No. 6,711,958, U.S. Pat. No. 6,920,798, U.S. Pat. No. 7,134,347, U.S. Pat. No. 7,392,709, or WO-A 03/027616.
Each of the measuring transducers includes a transducer housing, an inlet-side, first housing end of which is formed at least partially by means of a first flow divider having exactly two, mutually spaced, circularly cylindrical, or tapered or conical flow openings, and an outlet-side, second housing end formed at least partially by means of a second flow divider having exactly two, mutually spaced flow openings. In the case of some of the measuring transducers illustrated in U.S. Pat. No. 5,796,011, or U.S. Pat. No. 7,350,421, or US-A 2007/0151368, the transducer housing comprises a rather thick-walled, circularly cylindrical, tubular segment, which forms at least a middle segment of the transducer housing.
For guiding a medium (in given cases also an extremely hot medium), which flows, at least at times, the measuring transducers include furthermore in each case exactly two measuring tubes made of metal, especially steel or titanium, which are connected in such a manner that the medium can flow in parallel, and which are positioned within the transducer housing and held oscillatably therein by means of the aforementioned flow dividers. A first of the measuring tubes, which are most often equally constructed and extend parallelly relative to one another, opens with an inlet-side, first measuring tube end into a first flow opening of the inlet-side, first flow divider, and opens with an outlet-side, second measuring tube end into a first flow opening of the outlet-side, second flow divider. A second of the measuring tubes opens with an inlet-side, first measuring tube end into a second flow opening of the first flow divider, and opens with an outlet-side, second measuring tube end into a second flow opening of the second flow divider. Each of the flow dividers additionally in each case includes a flange with a sealing surface for fluid-tight connecting of the measuring transducer to tubular segments of the pipeline, which serve, respectively, for supplying and removing medium to and from the measuring transducer.
For producing the reaction forces discussed above, the measuring tubes are caused to vibrate during operation, driven by an exciter mechanism serving for producing or maintaining, as the case may be, mechanical oscillations, especially bending oscillations, of the measuring tubes in the so-called wanted mode. The oscillations in the wanted mode are, especially in the case of application of the measuring transducer as a Coriolis mass flow meter and/or densimeter, most often formed at least partially as lateral bending oscillations, and, in the case of medium flowing through the measuring tubes, as a result of Coriolis forces induced therein, superimposed with additional, equal-frequency oscillations in the so-called Coriolis mode. Accordingly, the—here most often electrodynamic—exciter mechanism is embodied in such a manner that, therewith, the two measuring tubes are, in the wanted mode, differentially—thus via introduction of exciter forces acting simultaneously along a shared line of action, but in opposed directions—excitable at least partially and especially also predominantly to opposite-equal bending oscillations.
For registering vibrations, especially bending oscillations, of the measuring tubes excited by means of the exciter mechanism, and for producing oscillation signals representing vibrations, the measuring transducers additionally each have a—most often likewise electrodynamic—sensor arrangement reacting to relative movements of the measuring tubes. Typically, the sensor arrangement is formed by means of an inlet-side oscillation sensor registering oscillations of the measuring tubes differentially—thus only relative movements of the measuring tubes—as well as by means of an outlet-side oscillation sensor registering oscillations of the measuring tubes differentially. Each of the oscillation sensors, which are usually equally constructed with respect to one another, is formed by means of a permanent magnet held on the first measuring tube and a cylindrical coil held on the second measuring tube and permeated by the magnetic field of the permanent magnet.
In operation, the above described tube arrangement formed by means of the two measuring tubes is, at least at times, excited by means of the electromechanical exciter mechanism to perform mechanical oscillations in the wanted mode at at least one dominating, wanted oscillation frequency. In such case, usually a natural, instantaneous resonance frequency of the tube arrangement, which, in turn, depends essentially both on the size, shape and material of the measuring tubes as well as also on an instantaneous density of the medium (and, insofar, as is known, can serve also as a measure for the density of the medium), is selected as the oscillation frequency for the oscillations in the wanted mode; in given cases, this wanted oscillation frequency can also be influenced significantly by an instantaneous viscosity of the medium. As a result of fluctuating density of the medium being measured and/or as a result of media change occurring during operation, the wanted oscillation frequency during operation of the measuring transducer varies naturally, at least within a calibrated, and thus predetermined, wanted frequency band, which correspondingly has a predetermined lower limit frequency and a predetermined upper limit frequency.
For defining a wanted oscillatory length of the measuring tubes and, in association therewith, for adjusting the band of the wanted frequency, measuring transducers of the above described type additionally most often include at least one inlet-side coupling element, which is affixed to both measuring tubes and spaced from the two flow dividers, this coupling element serving for forming inlet-side oscillation nodes for opposite-equal vibrations—especially bending oscillations—of both measuring tubes, as well as at least one outlet-side coupling element, which is affixed to both measuring tubes and spaced both from the two flow dividers, as well as also from the inlet-side coupling element, this outlet-side coupling element serving for forming outlet-side oscillation nodes for opposite-equal vibrations—especially bending oscillations—of the measuring tubes. In the case of curved measuring tubes, the length of a section of a deflection curve of any of the measuring tubes extending between the inlet side and the outlet-side coupling elements—and consequently the length of an imaginary center line of said measuring tube connecting the areal centers of gravity of all imaginary cross sectional areas of the respective measuring tube—corresponds to the wanted oscillatory length of the measuring tubes. By means of the coupling elements, which thus belong to the tube arrangement, also an oscillation quality factor of the tube arrangement, as well as also the sensitivity of the measuring transducer, can additionally be influenced, in a manner such that, for a minimum required sensitivity of the measuring transducer, at least one minimum, wanted oscillatory length is provided.
Development in the field of measuring transducers of vibration type has, by this point, reached a level wherein modern measuring transducers of the described type can, for a broad application spectrum of flow measurement technology, satisfy the highest requirements as regards precision and reproducibility of measurement results. Thus, such measuring transducers are, in practice, used for mass flow rates from some few g/h (grams per hour) up to some t/min (tons per minute), at pressures of up to 100 bar for liquids or even over 300 bar for gases. The accuracy of measurement achieved, in such case, usually lies at about 99.9% of the actual value or above, respectively a measuring error of about 0.1%, wherein a lower limit of the guaranteed measurement range can quite easily lie at about 1% of the measurement range end value. Due to their wide range of possibilities for use, industrial grade measuring transducers of vibration type are available with nominal diameters (corresponding to the caliber of the pipeline to be connected to the measuring transducer, or to the caliber of the measuring transducer measured at the connecting flange), which lie in a nominal diameter range of between 1 mm and 250 mm, and at maximum nominal mass flow rate of 1000 t/h, are in each case specified for pressure losses of less than 3 bar. A caliber of the measuring tubes lies, in such case, for instance, in a range of between 80 mm and 100 mm.
In spite of the fact that, by this point, measuring transducers for use in pipelines with very high mass flow rates and, in association therewith, very large calibers of far beyond 100 mm have become available, there is still considerable interest in obtaining measuring transducers of high precision and low pressure loss also for still larger pipeline calibers of about 300 mm or more, or mass flow rates of 1500 t/h or more, for instance for applications in the petrochemical industry or in the field of transport and transfer of petroleum, natural gas, fuels, etc. This leads, in the case of correspondingly scaled enlarging of the already established measuring transducer designs known from the state of the art, especially from EP-A 1 001 254, EP-A 553 939, US-A 2002/0157479, US-A 2007/0151368, U.S. Pat. No. 4,793,191, U.S. Pat. No. 5,370,002, U.S. Pat. No. 5,796,011, U.S. Pat. No. 6,308,580, U.S. Pat. No. 6,711,958, U.S. Pat. No. 7,134,347, U.S. Pat. No. 7,350,421, or WO-A 03/027616, to the fact that the geometric dimensions would be exorbitantly large, especially the installed length corresponding to a distance between the sealing surfaces of both flanges and, in the case of curved measuring tubes, a maximum lateral extension of the measuring transducer, especially dimensions for the desired oscillation characteristics, the required load-bearing ability, as well as the maximum allowed pressure loss. Along with that, also the empty mass of the measuring transducer increases unavoidably, with conventional measuring transducers of large nominal diameters already having an empty mass of about 400 kg. Investigations, which have been carried out for measuring transducers with two bent measuring tubes constructed, for instance, according to U.S. Pat. No. 7,350,421 or U.S. Pat. No. 5,796,011 as regards their to-scale enlargement to still greater nominal diameters, have, for example, shown that, for nominal diameters of more than 300 mm, the empty mass of a conventional measuring transducer enlarged to scale would lie far above 500 kg, accompanied by an installed length of more than 3000 mm, and a maximum lateral extension of more than 1000 mm. As a result, it can be said that industrial grade, mass producible measuring transducers of conventional design and materials with nominal diameters far above 300 mm cannot be expected in the foreseeable future both for reasons of technical implementability, as well as also due to economic considerations.
Taking this into consideration, in the assignee's own, non-pre-published international patent applications PCT/EP2010/068251, or PCT/EP2010/068250, for example, new measuring transducers of the vibration type, which are not least of all also scalable to comparatively large nominal nominal diameters of more than 300 mm, are in each case provided, wherein the particular tube arrangement of these measuring transducers in each case comprises four bent measuring tubes—for example, at least sectionally V-shaped and/or at least sectionally circular arc shaped measuring tubes—for conveying flowing medium. These measuring tubes are in each case connected to the flow dividers, which accordingly also each have four flow openings, so as to form flow paths, along which parallel flow can take place. Of the measuring tubes, a first measuring tube opens with an inlet-side first measuring tube end into in a first flow opening of the first flow divider, and opens with an outlet-side second measuring tube end into a first flow opening of the second flow divider; a second measuring tube parallel to the first measuring tube opens with an inlet-side first measuring tube end into a second flow opening of the first flow divider, and with an outlet-side second measuring tube end into a second flow opening of the second flow divider; a third measuring tube opens with an inlet-side first measuring tube end into a third flow opening of the first flow divider, and with an outlet-side second measuring tube end into a third flow opening of the second flow divider; and a fourth measuring tube parallel to the third measuring tube opens with an inlet-side first measuring tube end into a fourth flow opening of the first flow divider, and with an outlet-side second measuring tube end into a fourth flow opening of the second flow divider. Especially, in PCT/EP2010/068251, not least of all for the purpose of reducing undesired oscillation-related deformations of the flow dividers, it is additionally provided to excite as the wanted mode a natural bending oscillation mode inherent to the tube arrangement—the so-called V-mode—in which the first and second measuring tubes in each case execute opposite-equal bending oscillations about a static resting position associated with the respective measuring tubes, and in which the third and the fourth measuring tubes in each case execute opposite-equal bending oscillations about a static resting position associated with the respective measuring tubes, and in such a manner that, relative to an imaginary longitudinal section plane of the tube arrangement extending between the first and third measuring tubes as well as also between the second and the fourth measuring tubes, said bending oscillations of the first measuring tube are also opposite-equal to said bending oscillations of the third measuring tube, and that, relative to the imaginary longitudinal section plane, said bending oscillations of the second measuring tube are also opposite-equal to said bending oscillations of the fourth measuring tube.
Further investigations have revealed, however, that a special problem in such case can be that an eigenfrequency, thus also the instantaneous resonance frequency, of the V-mode of the tube arrangement is initially—thus without taking additional actions—essentially equal to an eigen-, respectively resonance, frequency of an additional natural bending oscillation mode inherent to the tube arrangement—the so-called X-mode—in which the first and second measuring tubes in each case execute opposite-equal bending oscillations about a static resting position associated with the respective measuring tubes, and in which the third and the fourth measuring tubes in each case execute opposite-equal bending oscillations about a static resting position associated with the respective measuring tubes, but do so, however, in such a manner that, relative to the aforementioned imaginary longitudinal section plane, said bending oscillations of the first measuring tube are also opposite-equal to said bending oscillations of the fourth measuring tube, and that, relative to the imaginary longitudinal section plane, said bending oscillations of the second measuring tube are also opposite-equal to said bending oscillations of the third measuring tube. As a result of this, in the case of measuring transducers with a tube arrangement of the aforementioned type having four measuring tubes, the case can certainly occur that the tube arrangement, when actually exciting the V-mode, namely to its instantaneous resonance frequency, switches in an unpredictable manner from the V-mode to the X-mode and vice versa, or simultaneously oscillates both in the V-mode as well as also in the X-mode, so that the tube arrangement, as a result, at times assumes undefined oscillatory states.